Bronchitis usually starts with a viral infection in your upper respiratory tract that moves down into your airways

Most bronchitis cases begin when a virus—usually the same ones that cause colds or flu—infects the lining of your windpipe and the tubes that branch into your lungs. Your body responds by producing mucus and triggering inflammation, which causes the cough that defines bronchitis. The infection typically takes hold over a few days, though you may not realize it's moved into your lower airways until the cough starts.

Acute bronchitis, the most common type, usually follows an upper respiratory infection. You might feel fine for a few days after a cold, then develop a persistent cough that lasts weeks. This is your body clearing the inflamed airways—uncomfortable but usually not dangerous on its own.

Chronic bronchitis, by contrast, develops over months or years from repeated irritation, most often from smoking. It's a permanent condition where your airways stay inflamed and produce excess mucus. Chronic bronchitis is actually one form of COPD (chronic obstructive pulmonary disease), not something you "catch" from a single infection.

Key Takeaways

  • Acute bronchitis usually follows a viral infection like a cold or flu, when the virus spreads from your upper airways down into your lungs.
  • You're most contagious during the first few days of illness, before the cough even starts, so you can spread it without knowing you have it.
  • Smoking, air pollution, and repeated exposure to irritants cause chronic bronchitis, which develops over time rather than from a single infection.
  • Bronchitis itself is not usually serious, but the cough can last three to eight weeks even after the infection clears.

How viruses trigger acute bronchitis

The viruses that cause bronchitis are the same ones circulating during cold and flu season. Rhinoviruses, influenza, parainfluenza, and respiratory syncytial virus (RSV) are the most common culprits. When you inhale respiratory droplets from a cough or sneeze, the virus lands in your nose or throat and begins to multiply.

For the first few days, you may have only cold symptoms—sore throat, runny nose, mild fatigue. The virus is replicating in your upper airways. Then it moves down into your bronchial tubes, the larger airways that carry air to your lungs. Your immune system responds with inflammation and mucus production, and that's when the cough begins. By this point, you've already been contagious for several days.

Bacterial bronchitis is less common but can develop after a viral infection weakens your airway defenses, or it can occur on its own. Bacteria like Streptococcus pneumoniae or Haemophilus influenzae may then colonize the inflamed tissue. This is why some people with bronchitis develop a secondary bacterial infection that requires antibiotics, though most cases remain viral.

Risk factors that make bronchitis more likely

Certain conditions and exposures increase your chances of developing bronchitis. Smoking is the single largest risk factor—it damages the protective lining of your airways and impairs the cells that clear mucus, leaving you vulnerable to infection and prone to chronic inflammation. Secondhand smoke exposure carries similar risk, particularly for children.

Age matters: very young children and adults over 50 develop bronchitis more often and tend to have more severe cases. A weakened immune system from HIV, recent chemotherapy, or immunosuppressive medications increases both your risk of infection and the likelihood that bronchitis will last longer or become complicated.

Environmental exposures also play a role. People who work with dust, chemical fumes, or grain dust—farmers, factory workers, construction workers—have higher rates of bronchitis. Air pollution in your city or neighborhood increases risk, as does living in a home with poor ventilation or mold. Acid reflux can also trigger bronchitis by allowing stomach acid to irritate your airways.

How bronchitis spreads from person to person

Bronchitis spreads the same way colds and flu do: through respiratory droplets. When an infected person coughs or sneezes, they release tiny droplets containing virus into the air. You inhale these droplets, and the virus enters your respiratory tract. You can also catch it by touching a contaminated surface—a doorknob, phone, or countertop—and then touching your face.

You're most contagious during the first three to five days of illness, when the viral load in your respiratory tract is highest. This is often before you develop the characteristic cough, so you can spread bronchitis without realizing you have it. Once the cough starts, you remain contagious but typically less so than in those early days.

Crowded indoor spaces increase transmission risk: schools, offices, public transit, and healthcare settings are common places where bronchitis spreads. Winter months see more cases because people spend more time indoors in close quarters, and respiratory viruses circulate more widely during cold season.

Why some people develop chronic bronchitis instead

Chronic bronchitis develops when your airways are repeatedly irritated over months or years, not from a single viral infection. Smoking is the overwhelming cause—the heat and chemicals in tobacco smoke damage the cells lining your bronchial tubes and disable the cilia, the tiny hair-like structures that sweep mucus out of your lungs. Without functioning cilia, mucus accumulates, and your body produces even more in response to the irritation.

Long-term exposure to occupational dust or fumes can cause chronic bronchitis even in people who've never smoked. Miners, grain handlers, and metal workers face this risk. Severe air pollution in your area, particularly fine particulate matter (PM2.5), can also lead to chronic airway inflammation over time.

Chronic bronchitis is defined by a persistent cough that produces mucus on most days for at least three months per year, for two consecutive years. Once the damage is done, it doesn't reverse—chronic bronchitis is a long-term condition requiring ongoing management, unlike acute bronchitis, which resolves as the infection clears.

What happens in your airways during bronchitis

When infection or irritation triggers bronchitis, the lining of your bronchial tubes becomes inflamed and swollen. Blood vessels in the airway walls dilate, and fluid leaks into the tissue, causing further swelling. Your immune system releases inflammatory chemicals, and specialized cells in the airway lining begin producing excess mucus as a defense mechanism.

This combination—swelling, mucus, and inflammation—narrows your airways and irritates the nerve endings in the bronchial tubes. That irritation triggers the cough reflex. Your body is trying to clear the mucus and irritants, but the cough often becomes persistent and exhausting because the inflammation takes time to resolve, even after the virus is gone.

In acute bronchitis, this process typically lasts two to three weeks for the infection itself, though the cough can linger for six to eight weeks as the inflammation gradually subsides. In chronic bronchitis, the inflammation never fully resolves—it becomes the baseline state of your airways, and the cough becomes a permanent feature of daily life.

Frequently Asked Questions

Can you get bronchitis from cold air or weather changes?

Cold air doesn't cause bronchitis directly, but it can trigger coughing in people who already have airway inflammation. Cold, dry air irritates inflamed airways and can worsen symptoms. However, the underlying infection or irritation must already be present—weather alone won't give you bronchitis.

Is bronchitis contagious after you start coughing?

Yes, you remain contagious while you're coughing, though you're most contagious in the first few days before symptoms appear. You can spread it to others for at least the first week of illness, possibly longer. Covering your cough and washing your hands reduces transmission risk.

Can you get bronchitis more than once?

Yes. Each time you're exposed to a new respiratory virus, you can develop acute bronchitis. People who smoke or have chronic lung disease are more prone to repeated episodes. Chronic bronchitis, by contrast, is a persistent condition that doesn't go away between episodes.

Does bronchitis always follow a cold?

Bronchitis often follows a cold, but not always. You can develop bronchitis from any respiratory virus, including flu or RSV, and sometimes from bacterial infection. Some people develop bronchitis without remembering a preceding cold—the upper respiratory symptoms may have been mild or overlooked.

Can allergies cause bronchitis?

Allergies don't cause bronchitis directly, but they can increase your risk. Allergic inflammation in your airways makes them more vulnerable to viral infection. People with asthma or allergic rhinitis have higher rates of bronchitis because their airways are already irritated and reactive.